If a range of forces are used to extend a spring, it undergoes elastic deformation and the force-extension graph is a straight line through the origin up to a limit.This means that the force is directly proportional to the extension up to the limit of proportionality. If the force is removed, the spring returns to its original length.If the spring is stretched further it reaches its elastic limit. At this point the spring undergoes plastic deformation and does not return to its original length when the force is removed.
Hooke's Law states that the extension of a spring is directly proportional to the force applied provided the limit of proportionality is not exceeded.